Mixing device for a fuel reformer, fuel reformer and method for converting hydrocarbon fuels into hydrogen rich gas

ABSTRACT

A mixing device for a fuel reformer for mixing at least two fluids is provided. The mixing device includes at least a first plurality of holes which is arranged along a first row, and a second plurality of holes which is arranged along a second row. The mixing device can be used in a fuel reformer for converting hydrocarbon fuel into hydrogen rich gas by auto-thermal reaction process having a, preferably cylindrically shaped and double walled, housing with two side walls forming a reaction chamber of the fuel reformer, wherein hydrocarbon fuel and an oxidizing agent are mixed by the mixing device.

The present invention relates to a mixing device for a fuel reformer, afuel reformer for converting hydrocarbon fuels into hydrogen rich gasfor fuel cells and/or exhaust treatment applications comprising such amixing device, and a method for converting hydrocarbon fuels intohydrogen rich gas using the fuel reformer.

In the state of the art it is well known to produce hydrogen rich gasfor the use in fuels cells of transportation devices by reforminghydrocarbon fuels, like gasoline or hydrocarbon fuels. Conventionally,hydrogen is produced in large-scale industrial facilities and thenstored on board of the transportation devices. The recent development ofsmall-scale on-board hydrogen sources, so called fuel reformers,provides a possibility for producing hydrogen on demand without thenecessity of hydrogen storage.

In general, there are three known methods of reforming gaseous or liquidhydrocarbon fuels into hydrogen: catalytic steam reforming, partialoxidation reforming and auto-thermal reforming. In all known methods twofluids, namely hydrocarbon fuel and an oxidizing agent, such as steam,air or water, are mixed and supplied to a catalyst arranged in areaction chamber of a fuel reformer.

In catalytic steam reforming processes, a mixture of steam andhydrocarbon fuel is exposed to a suitable catalyst, like nickel, at ahigh temperature (typically between 700° C. and 1000° C.). The reactionis highly endothermic and requires an external source of heat and asource of steam.

In partial oxidation reforming processes, a mixture of hydrogen fuel andan oxygen containing gas, like ambient air, are fed as feed gas into areaction chamber, preferably in the presence of a catalyst. The catalystused is normally made from a noble metal or nickel, and the temperatureis typically between 700° C. and 1700° C. The reaction is highlyexothermic and, once started, it generates sufficient heat to be selfsustaining. In order to promote the oxidation reaction, it is necessaryto reduce temperature variations in the reaction chamber.

Auto-thermal reforming processes are a combination of steam reformingand partial oxidation reforming. Waste heat from the partial oxidationreforming reaction is used to heat the endothermic steam reformingreaction.

The natural by-products of all reforming processes are carbon monoxideand carbon dioxide. But, since the hydrocarbon fuels were not designedas a feed stock for generating hydrogen, there are also otherby-products such as sulphur. These byproducts may be harmful to the fuelcells and should therefore be removed by subsequent steps outside thefuel reformer. Additionally, hydrocarbon fuels, especially diesel, canproduce soot as a by-product in the catalyst, if the mixing in thereaction chamber is poor. Soot particles again, are very harmful to thefuel cells, and care must be taken to avoid the formation of soot in thefuel reformer.

From the state of the art, U.S. Pat. No. 6,770,106, a partial oxidizingfuel reformer for reforming feed gas containing hydrocarbon or methane,oxygen and water, is known, wherein the reduction of temperaturevariations is achieved by a reaction chamber being covered with apassage for feed gas, which is heated by the reaction heat in thereaction chamber and therefore thermally isolates the reaction chamber.

Thereby, temperature variations inside the reaction chamber can bereduced. For heating the feed gas, the reaction heat can be recovered bya heat exchanger.

Additionally, it has been found that a successful and efficientconversion of the feed gas into a hydrogen rich gas is dependent on asuccessful mixing of the fluids or reactants, namely hydrocarbon fueland the oxidizing agent. The disadvantage of the known state of the artis that, since the mixing of the reactants is performed in a further,externally arranged mixer, a perfect atomization or vaporization of thehydrocarbon fuel and the oxidants without condensation of the fuel inthe reaction chamber of the fuel reformer cannot be provided.

For solving this problem it has been proposed in the state of the arte.g. to mix hydrocarbon fuel and oxidizing agent in the reaction chamberand, preferably, even to vaporize injected hydrocarbon fuel bypreheating the incoming air stream to be mixed with the fuel, or bypreheating a reaction chamber surface for receiving a fuel spray. Noneof the prior art approaches is entirely successful in providing areliable, complete vaporization of the injected hydrocarbon fuel and ahomogenous mixing of the hydrocarbon fuel and the oxidizing agent. Themain problem arising by the mixing of hydrocarbon fuel and an oxidizingagent is to provide a homogenous gaseous mixing of the two fluids,wherein at least one of the fluids, particularly hydrocarbon fuel, isprovided in its liquid phase.

Another disadvantage of the mixing as performed in the state of the artis the generation of a recirculation of the fluid streams. Thisrecirculation is generated, as the fluid flow, which is provided by theinjection of hydrocarbon fuel and/or of the oxidizing agent into thereaction chamber, hits the catalyst arranged in the reaction chamber andis repelled therefrom. This recirculation can even produce a backflow ofthe fluids through the fluid inlets.

Therefore, it is object of this invention to provide a mixing device fora fuel reformer, a fuel reformer and a method for converting hydrocarbonfuels into hydrogen rich gas, wherein a higher degree of homogenousmixing of the reactants is provided and recirculation is reduced.

It has been discovered by the inventor that the recirculation of thefluids can be reduced by using a mixing device, which has at least afirst plurality of holes arranged along a first row and a secondplurality of holes arranged along a second row. By means of the firstand the second rows of holes fluid jets are provided downstream of themixing device. By means of the generated fluid jets, recirculation canbe controlled and the mixing of said fluids be improved. The shape ofthe rows preferably resembles the shape of an outer edge of the mixingdevice, whereby the circumference of the first row advantageously issmaller than the circumference of the second row. Thereby, the mixingdevice establishes a so called plug flow, which streams almost planarand substantially in parallel to a longitudinal axis of a reactionchamber, the mixing device can be mounted in or at.

Preferably, the mixing device is designed such that the number, the sizeand/or the arrangement (rows) of the holes of the first and the secondplurality of holes is/are optimized for a shape and/or a size of a fuelreformer the mixing device is designed for.

In another preferred embodiment of the mixing device, the mixing deviceis designed such that the holes of the first plurality of holes arearranged in flow direction of the oxidizing agent streaming through e.g.oxidizing agent inlets of a reaction chamber of a fuel reformer. It isfurther advantageous to arrange the holes of the second plurality ofholes, preferably alternating to the holes of the first plurality ofholes, at the periphery of the mixing device.

In this preferred embodiment of the mixing device, the number of holesof the first plurality of holes is equal to the number of holes of thesecond plurality of holes. By this arrangement, the mixing device can bedivided into segments, wherein the segments either comprise a hole ofthe first plurality of holes or a hole of the second plurality of holes.The size of the segments preferably depends on the overall size of themixing device and the overall number of holes.

plurality of holes and/or the diameter of the second plurality of holesis preferably defined by the overall number of holes and the size of themixing device.

According to a further preferred embodiment of the mixing device, themixing device has a preferably centrally located opening, which isadapted to encompass, preferably in a sealing manner, a fuel injectionelement of the fuel reformer. Through this fuel injection element,preferably preheated hydrocarbon fuel is sprayed into the reactionchamber of such a fuel reformer, where it mixes with the oxidizing agentstreaming through the holes of the mixing device.

Preferably, the holes of the first plurality of holes are arrangedaround the centrally located opening. Since hot oxidizing agent streamsthrough the holes of the first plurality of holes, its waste-heatsupports the vaporization of the hydrocarbon fuel.

For increasing the homogeneity of the mixing, it is further preferred toprovide at least one hole of the second plurality of holes with ashroud. These shrouds provide a swirling motion of the oxidizing agentdownstream of the mixing device which further increases the turbulenceof the fluid jets and thereby improves the homogenous mixing of thehydrocarbon fuel and the oxidizing agent.

In a further embodiment of the mixing device, the ratio of the diameterof one of the first plurality of holes to the diameter of the first rowis proportional to tan(β/2) and/or the ratio of the diameter of one ofthe second plurality of holes to the diameter of the second row isproportional to tan(α/2), wherein angle β defines a first type ofsegment and angle α defines a second type of segment of the mixingdevice. Angle α defines the size of a segment comprising one hole of thesecond plurality of holes and angle β defines the size of a segmentcomprising one hole of the first plurality of holes. The angles α and βand the number of holes are interrelated, wherein the angles α and β canbe equal and/or relate to the number of holes by the equationn₁·α+n₂·β=360°, with n1 being the number of holes of the first pluralityof holes and n2 being the number of holes of the second plurality ofholes. The invention is not limited to the case of two such rows with aplurality of holes each being arranged at the mixing device. There couldbe even three or more rows of such pluralities of holes being arrangedat the mixing device.

In case there is for instance a third row of holes defining a thirdplurality of holes, the size of the segment comprising one hole of thethird plurality of holes would be defined by an angle y. In this case,the segments and number of holes would relate to each other by theequation n₁·α+n₂·β+n₃·γ=360° with n1 being the number of holes of thefirst plurality of holes, n2 being the number of holes of the secondplurality of holes and n3 being the number of holes of the thirdplurality of holes. Thus, in general, segment size and number of holesrelate to each other according to the formula

${\sum\limits_{i}^{{Number}\mspace{14mu}{of}\mspace{14mu}{rows}}{n_{i} \cdot \alpha_{i}}} = {360{{^\circ}.}}$

According to a further preferred embodiment of the mixing device, themixing device has a circular shape and the first and/or second row/s ofholes are/is preferably formed as circle/s, too. Advantageously, theshape of the mixing device is defined by the reaction chamber it ismounted in or at. With other words, if the reaction chamber has aquadratic cross section, also the mixing device is preferably quadratic,and if the reaction chamber is tubular shaped (i.e. circular orelliptical shaped), as most fuel reformers are, the mixing device haspreferably a tubular, i.e. circular or elliptic shape, too.

It is further advantageous to provide a fuel reformer having a reactionchamber, wherein the above described mixing device according to theinvention is arranged in a sealing manner inside the reaction chamber.In such an arrangement, the mixing device provides a substantiallyplanar fluid flow which corresponds to an axial fluid flow along thelongitudinal axis of the fuel reformer. Since the planar fluid flow isdesigned to use the whole cross section of the reaction chamber, fluidmainly streams in a controllable (and controlled) manner in onedirection through the reaction chamber, whereby a recirculation of thefluid flow is reduced.

In a further preferred embodiment of the fuel reformer, the mixingdevice is arranged near a fuel inlet side of the fuel reformer and theoxidizing agent inlets are preferably arranged between the fuelinjection inlet side of the fuel reformer and the mixing device. By thisarrangement a pressure is established in the space between the fuelinjection inlet side and the mixing device which forces the oxidizingagent to flow through the holes of the mixing device in downstreamdirection, which further reduces the risk that unwanted backflow orrecirculation of the fluid stream occur.

Preferably, the oxidizing agent inlets form slits which are arrangedalong the circumference of the fuel reformer. Advantageously, the slitsor the mixing device are designed such that the slits and the holes ofthe second plurality of holes of the mixing device are arranged off-linerelative to each other.

This off-line arrangement of the holes of the second plurality of holesand the slits ensures that during the start up phase the oxidizingagent, preferably even a major part thereof, streaming into the reactionchamber of the fuel reformer also reaches the holes of the firstplurality of holes at the mixing device. If the slits were to beradially arranged in line with the holes of the second plurality ofholes of the mixing device, the major part of oxidizing agent wouldstream directly through the holes of the second plurality of holes. Thisin turn would impair the establishment of a plug flow already during thestart-up phase. After having reached a steady state, the oxidizing agentstreams through both the first plurality of holes and the secondplurality of holes, wherein due to smaller diameter of the holes of thefirst plurality of holes, the major part of the oxidizing agent streamsthrough the second plurality of holes.

Any of the embodiments of the mixing device described above can bemounted inside the reaction chamber of such fuel reformer according tothe invention.

According to another aspect of the invention regarding the fuelreformer, a substantially complete fuel atomization and subsequent gasmixture can be achieved by pre-heating the hydrocarbon fuel beforeintroducing the hydrocarbon fuel into the fuel reformer, in particularinto the mixing device arranged in such fuel reformer, and mixing itthere with the oxidizing agent. Such a preheating can be achieved bypreheating means which could be arranged outside of the reaction chamberof the fuel reformer. The preheating means can be a separate devicebeing arranged upstream to a fuel inlet, but it is also possible tointegrate the fuel inlet and the preheating means into a single device.Particularly, in the preferred case, where a fuel injector is used asfuel inlet, it is advantageous to heat the injector, whereby the fuel ispreheated, too. Further, in another preferred embodiment the fuel inletis in heat conductive contact with a side wall of the reaction chamberand/or the mixing device, which in turn makes it possible to transferthe heat generated in the reaction chamber to the injector forpreheating the fuel.

In a further preferred embodiment of the fuel reformer, the temperatureof the preheated fuel is adapted to be close to, but below the lowestboiling point of the fuel, whereby a fuel temperature required forsubstantially complete atomization or vaporization is provided.

According to a further advantageous embodiment of the fuel reformer,also the oxidizing agent is preheated prior to the mixing with thehydrocarbon fuel, preferably to a temperature in the same range orhigher than the temperature of the preheated fuel. This substantiallyprevents unwanted condensation of the fuel or the oxidizing agent, whichcould result in a shortened life time of the fuel reformer.

The preheating of the oxidizing agent can be preferably performed byusing a fuel reformer having an inner wall and an outer wall forming aspace in-between, wherein said space is designed as oxidizing agentpassage between an oxidizing agent supply port provided in or at theouter wall of the fuel reformer and an oxidizing agent inlet provided inor at the inner wall of the fuel reformer. The inner wall is heated bythe heat of the chemical reactions taking place inside the reactionchamber, whereby in turn the oxidizing agent is preheated by the innerwall by heat transfer from the inner wall to the oxidizing agent. Anadvantageous side effect of the heating of the inner wall is that incase a spray of liquid fuel comes into contact with the inner wall, theheat provided by the inner wall supports evaporation of the liquid fuel.

According to a further preferred embodiment of the fuel reformer, thepreheating of the oxidizing agent is additionally or alternativelyperformed by an externally arranged heater. This is particularlyadvantageous during a start-up phase of the fuel reformer, where thereaction chamber needs to be heated up to its normal operatingtemperature. Due to the double walled structure provided by thearrangement of the oxidizing agent passage, the heat of the preheatedoxidizing agent can be provided to the inner wall and thereby to thereaction chamber, whereby the time required for the heating up of thefuel reformer can be reduced.

The combination of fuel preheating and mixing the atomized fuel with theoxidizing agent downstream of the mixing device results in asubstantially completely homogenous reactant mixture, and preheating ofthe oxidizing agent reduces unwanted condensation effects. The achievedsubstantially homogenous mixture allows for a substantially completeconversion of the hydrocarbon fuel which in turn allows for an efficientproduction of hydrogen rich gas from converted heavy hydrocarbon fuelwhich in turn is a prerequisite for the subsequent production of fuelcell grade hydrogen.

An advantageous side effect of the above described preheating of theoxidizing agent by heat transfer from the inner wall of the reactionchamber to the oxidizing agent is that the heat transfer also cools theinner wall to a temperature, at which the formation of soot by burningof fuel particles coming in contact with the inner walls issubstantially reduced.

Generally, there is the possibility of providing the reaction chamberwith an external cooling device but this increases the dimensions of thereaction chamber and adds a further consumer of energy to the system(that has to be supplied with energy produced e.g. by the fuel cells).Therefore, it is usually more advantageous to use the relatively cooloxidizing agent for cooling the inner wall of the reaction chamber. Sucha solution has the further advantage that a thermal isolation of theinner wall is in most applications not necessary, whereby the overalldimensions of the fuel reformer can be further reduced.

Another advantage of the cooling of the inner wall by the oxidizing,agent is that the temperature inside the reaction chamber can be heldsubstantially constant, and that the temperature of the oxidizing agentcan be controlled.

As shown in another preferred embodiment of the fuel reformer, theoxidizing agent inlet provided in the inner wall of the housing isformed as a plurality of orifices, particularly holes or minute slits.Preferably, size, shape and/or location of the orifices can varyaccording to the design of the used mixing device, the used oxidizingagent, the used hydrocarbon fuel and/or their temperature. Mostpreferably, the oxidizing agent inlet is provided in the vicinity of thefuel inlet and between the fuel inlet side of the fuel reformer and themixing device.

Another preferred embodiment of the fuel reformer is provided with acatalyst for the auto-thermal reaction inside the reaction chamber ofthe fuel reformer in order to accelerate the conversion of hydrocarbonfuel into hydrogen rich gas. Since the mixing is performed in accordancewith the inventive method, a substantially completely homogenous mixtureof the hydrocarbon fuel and the oxidizing agent can be brought intocontact with the catalyst. Further, the substantially reducedcondensation effects of the mixture or one of the components of themixture achievable with the invention reduces substantially the risk ofdeactivation of the catalyst and thereby prolongs the life-time of thefuel reformer. Preferably, the catalyst can be a ceramic monolith or ametal grid.

Preferably, the distance between the mixing device and the catalyst inthe reaction chamber of the fuel reformer is also constructed such thatthe oxidizing agent achieves mixture stabilization without causingauto-oxidation of the oxidizing agent/fuel mixture.

Further, a method for converting hydrocarbon fuel into a hydrogen richgas according to the invention is provided, wherein one of the abovedescribed preferred embodiments of the fuel reformer is used forconverting hydrocarbon fuel into hydrogen rich gas. The use of a fuelreformer comprising an embodiment of the inventive mixing deviceprovides a homogenous mixing of the hydrocarbon fuel and the oxidizingagent. The achieved homogenous mixture allows for a substantiallycomplete conversion of the hydrocarbon fuel which in turn allows for anefficient production of hydrogen rich gas from converted heavyhydrocarbon fuel which in turn is a prerequisite for the subsequentproduction of fuel cell grade hydrogen.

Other preferred embodiments and advantages are also provided.

In the following, preferred embodiments of the mixing device and thefuel reformer according to the invention will be discussed with help ofthe attached Figures. The description is considered as exemplificationof the principles of the invention and is not intended to limit thescope of the claims.

The Figures show:

FIG. 1: a schematic view of a preferred embodiment of said fuel reformeraccording to the invention;

FIG. 2: a schematic perspective view of the fuel reformer shown in FIG.1;

FIG. 3: a perspective view of a section of a preferred embodiment of themixing device according to the invention; and

FIG. 4: a schematic view of the dimensions of a preferred embodiment ofthe mixing device according to the invention.

The fuel reformer 1 in FIG. 1 comprises a housing 2 with an inner wall4, an outer wall 6 and side walls 8 a, 8 b. The housing 2 has a circularcross section (in relation to the longitudinal axis of the fuel reformer1 stretching between the sidewalls 8 a and 8 b). Other forms of thecross section as for instance an elliptical or a quadratic orrectangular cross section are possible as well. Inside the housing 2,hydrocarbon fuel 10 and an oxidizing agent 12 are brought into contactwith each other by means of a mixing device 14 so that a preferablyauto-thermal reaction can take place in a reaction chamber 16 defined bythe inner wall 4, the mixing device 14 and the side wall 8 b.

The inner wall 4 and the outer wall 6 of the fuel reformer 1 define aspace 18 between them. The space 18 in turn forms a passage for theoxidizing agent 12 between an oxidizing agent supply port 20 and one ormore oxidizing agent inlets 22.

Additionally, the fuel reformer 1 includes a catalyst 24 for catalyzingthe auto-thermal reaction in the reaction chamber 16. The catalyst 24accelerates the auto-thermal reaction, but it is also possible to use afuel reformer according to the present invention without such acatalyst. The catalyst 24 is preferably a metal grid or a ceramicmonolith, but it is possible to use any other suitable substrate for thedesign of the catalyst 24.

The oxidizing agent inlet 22 is formed as a plurality of orifices,particularly as holes and/or minute slits, the number, size, shape andlocation of which vary depending on the used mixing device 14, the usedoxidizing agent 12, the used hydrocarbon fuel 10 and the temperature ofthese fluids. The plurality of orifices 22 can have uniform size andshape, but it is also possible that the orifices vary in size and shapeamong each other. Preferably, the orifices are designed as slits 22which are substantially equidistant, wherein also the length of theslits 22 is substantially equal to the distance between adjacent slits22.

The mixing device 14 comprises a first plurality of holes 26 and asecond plurality of holes 28, which are arranged along rows, preferablyin form of concentric circles. The diameter of the circle of the firstplurality of holes 26 is smaller than the diameter of the circle of thesecond plurality of holes 28. The oxidizing agent 12 streams through theslits 22 into a space 30, where a pressure will be generated by theincoming oxidizing agent 12, and is subsequently forced by said pressurethrough thβ first and the second plurality of holes 26, 28 in the mixingdevice 14 into the reaction chamber 16, where it mixes with thehydrocarbon fuel 10 that is injected into the reaction chamber 16 via ahydrocarbon fuel inlet 32. The design of the mixing device 14 will beexplained in detail with reference to FIG. 2 to 4 below.

The distance L between the mixing device 14 and the catalyst 24 is alsoconstructed in such a way that mixture stabilization of the oxidizingagent/fuel mixture is achieved by the oxidizing agent 12 without causingauto-oxidation of the oxidizing agent/fuel mixture.

Further, the hydrocarbon fuel inlet 32 at the fuel reformer is locatedin the side wall 8 a of the housing 2 and extends through the mixingdevice 14 to the reaction chamber 16. Preferably, the hydrocarbon fuelinlet 32 is formed as a fuel injector which provides a fuel spray in thereaction chamber 16.

A reformer gas outlet 34 is provided in the opposite side wall 8 b ofhousing 2. The reformer gas 36 is a hydrogen rich gas which can be usedfor operating fuel cells (subject to any necessary further processing asfor instance cleaning and purification) and is the product of theauto-thermal reaction in the fuel reformer 1.

As shown in FIG. 1, the fuel reformer 1 further comprises a preheatingmeans 38 for preheating the hydrocarbon fuel 10. In FIG. 1, the fuelpreheating means 38 is illustrated as separate device, but it is alsopossible to integrate the fuel injector 32 and the fuel preheating means38 into a single device. If the fuel injector 32 is additionally in heatconductive contact with the side wall 8 a and/or the mixing device 14,heat generated in the reaction chamber 16 can be transferred to the fuelinjector 32, where it can be used to preheat the hydrocarbon fuel 10.

In the following the operation of the fuel reformer 1 is described bymeans of the exemplary conversion of hydrocarbon fuel into hydrogen richgas with an air/steam-mixture as oxidizing agent. The reaction for theconversion is auto-thermal.

According to the invention, air and steam are mixed before theair/steam-mixture 12 is injected by oxidizing agent supply port 20 intospace 18, which serves as air/steam passage for transportation of theair/steam mixture 12 from the oxidizing agent supply port 20 to theoxidizing agent inlet 22 of the fuel reformer 1.

Dependent on the kind of reforming process (partially oxidizing,auto-thermal or steam reforming process), the direction of heat transferbetween the air/steam mixture 12 and the inner wall 4 differs.

For auto-thermal or steam reforming processes for instance, preferablythe air/steam mixture 12 is preheated by an external heating device (notshown), so that the hot air/steam mixture 12 can transfer heat to theinner wall 4. Thereby, particularly during the start-up phase, thereaction chamber 16 can easily be brought to, and kept at, its normaloperating temperature.

In partial oxidation reforming processes, preferably, theair/steam-mixture 12 is preheated on the way to the slits 22 in theinner wall 4 of the housing 2 by heat transfer from the inner wall 4 tothe air/steam mixture 12, whereby the heat transfer also cools the innerwall 4 of reaction chamber 16. By cooling the inner wall 4 of thereaction chamber 16, also the risk will be reduced that hydrocarbon fuelmolecules in the reaction chamber 16 is being burned to soot whenhitting the reaction chamber wall. The inner wall 4 of the reactionchamber 16 is heated by the substantially homogenous oxidation takingplace in the reaction chamber 16 when oxygen from the air/steam-mixture12 reacts with “lighter” hydrocarbon molecules of the hydrocarbon fuel10 having shorter chains (CxHy+O2→CO2+CO+H2O).

During the start-up phase of the fuel reformer 1, oxidizing agent 12 asfor instance the air/steam mixture 12 preheated by an external heater(not shown) can be supplied to the oxidizing supply port 20, whichtransfers its heat to the inner wall 4 and the catalyst 24, whereby thetime for bringing the fuel reformer 1 to its normal operatingtemperature can be reduced.

The air/steam mixture 12 streams through the slits 22 into the space 30,where a pressure is generated which forces the air/steam mixture 12through the holes of the mixing device 14 into the reaction chamber 16of the fuel reformer 1. Thereby, a substantially homogenous air/steammixture is formed in the reaction chamber 10, where it is mixed withhydrocarbon fuel 10 that is sprayed into the air/steam mixture by meansof fuel injector 32.

For a successful mixing of the hydrocarbon fuel 10 and the air/steammixture 12 a substantially perfect atomization or vaporization of thehydrocarbon fuel 10 into the air/steam mixture 12 is required in orderto substantially keep any unwanted condensation of the hydrocarbon fuel10 or the air/steam mixture 12 at a tolerable minimum. Since such anunwanted condensation likely occurs due to temperature differencesbetween the preheated air/steam mixture 12 and the normally coolerhydrocarbon fuel 10, according to the embodiment of the invention shownin FIG. 1, also the hydrocarbon fuel 10 is preheated by the preheatingmeans 38.

A substantially perfect fuel atomization or vaporization of thehydrocarbon fuel 10 and a subsequent mixture of the atomized orvaporized hydrocarbon fuel 10 with the air/steam mixture 12 is achievedby preheating the hydrocarbon fuel 10 to a temperature close to, butbelow the lowest boiling point of the hydrocarbon fuel 10, whereby alsoheat for a substantially complete atomization or vaporization isprovided.

Preferably, also the air/steam mixture 12 is preheated to a temperaturein the same temperature range or higher than the temperature of thehydrocarbon fuel 10, whereby an elevated temperature between thehydrocarbon fuel 10 and the air/steam mixture 12 is provided, which inturn substantially prevents condensation of the substances or at leastkeeps it at a tolerable minimum.

Since hydrocarbon fuel, and particularly diesel fuel, is a mixture ofdifferent components, whereby each of which has a different boilingpoint, the air/steam mixture 12 is preferably at least preheated to atemperature higher than the boiling point of the lightest components ofthe hydrocarbon fuel 10 which defines the lowest boiling point of thehydrocarbon fuel 10. However, it is more advantageous, to preheat theair/steam mixture 12 to a temperature that is higher than the highestboiling point of the hydrocarbon fuel 10. By preheating the air/steammixture 12 to such a temperature a substantially complete vaporizationof the hydrocarbon fuel 10 can be achieved.

The combination of fuel preheating and mixing the atomized hydrocarbonfuel 10 with the air/steam mixture 12 results in a substantiallycompletely homogenous reactant mixture that allows for substantiallycomplete conversion of the hydrocarbon fuel 10 into a hydrogen rich gas36 which in turn allows for an efficient production of fuel cell gradehydrogen.

In order to provide a turbulent and homogenous mixture of the oxidizingagent 12 with the hydrocarbon fuel spray 10, the second plurality ofholes 28 can further include shrouds which are designed to generate aswirling motion of the oxidizing agent 12 streaming through these holes.As a result, a substantially completely homogenous mixture ofhydrocarbon fuel 10 and the oxidizing agent 12 is generated before thismixture comes into contact with the catalyst 24.

This substantially homogeneous gas mixture is then pushed through thecatalyst 24, where the hydrocarbons of the hydrocarbon fuel 10 areundergoing the auto-thermal reaction process. In the auto-thermalreaction process taking place inside the catalyst hydrogen H2, CO andCO2 are produced as major process end products. These end products areprocessed in subsequent steps outside the fuel reformer 1 with the aimto separate H2 from all other process end products to such a degree thatfuel cell grade hydrogen eventually is provided.

FIG. 2 shows a schematic perspective view of the reformer 1 illustratedin FIG. 1 from a perspective view from the fuel inlet side wall 8 a,wherein the side wall 8 a has been removed. As can be seen from theFigure, the mixing device 14 is arranged in a sealing manner inside thetubular shaped cylindrical reformer 1 and is mounted at the inner wall 4of the housing 2 of the fuel reformer 1. As explained above, theoxidizing agent 12 streams through the space 18 formed between the innerwall 4 and the outer wall 6 of the housing 2 and enters the mixingdevice 14 in radial direction through the slits 22, which are arrangedbetween the side wall 8 a (not shown) and the mixing device 14.

The mixing device 14 has at least a first row of holes 26, which arepreferably arranged along a first circle, and a second row of holes 28,which are preferably arranged along a second circle. Both circles areconcentric to each other, and the first or inner circle has a smallerdiameter than the second or outer circle. Preferably, the number of thefirst respectively second plurality of holes is equal, and the holes arearranged in an alternating manner that will be explained more in detailin connection with FIG. 4 further below.

It should be noted that the shape of the mixing device 14 resembles thecross-section of the fuel reformer 1. Therefore, a cylindrical ortubular fuel reformer with a circular or elliptical cross sectionresults in a circular or elliptic shape of the mixing device. But it isalso possible that reformer and mixing device have different shapes e.g.squared, rectangular or poly-angular.

The overall number of holes in the mixing device 14 also defines thediameter of the holes of the first plurality of holes 26 and thediameter of the holes of the second plurality of holes 28. The relationbetween the size of the holes, the radius of the circles and the numberof holes will be explained with reference to FIG. 4 further below.

During the start-up phase, a major part of the oxidizing agent 12 (asfor instance air/steam mixture) streams, after having entered the space30 through slits 22, in radial direction to the first plurality of holes26 and through these holes 26 as hot gas. Since the oxidizing agent 12disperses in the space 30, a part of it also flows off through the holes28 of the second plurality of holes, which are arranged along the outeredge of the mixing device 14. Since the plurality of holes 26 and 28have effectively a smaller overall opening than the overall openingestablished by the plurality of slits 22, in the start-up phase of theoperation of the fuel reformer 1 more oxidizing agent 12 flows into thespace 30 than is drained off through the mixing device 14 via the holes26 and 28. Therefore, the space 30 is gradually filled with oxidizingagent 12 and a pressure is established that forces the oxidizing agent12 through the holes 26 and 28. In the steady-state, when the sameamount of oxidizing agent 12 flows into the space 30 as is drained offthrough the mixing device 14, the oxidizing agent 12 streams throughboth the first plurality of holes 26 and the second plurality of holes28, wherein due to smaller diameter of the holes of the first pluralityof holes 26, the major part of the oxidizing agent 12 streams throughthe second plurality of holes 28.

In the centre of the mixing device 14 a receiving opening 40 is arrangedwhich is adapted to accommodate, in a sealing manner, the fuel injectionelement 32.

Preferably the receiving opening 40 has the same shape as, the fuelinjection element 32.

Since the holes 28 of the second plurality of holes are off-set from theradial main stream direction of oxidizing agent 12 provided by thearrangement of the slits 22, it is ensured that the oxidizing agent 12streams also through the first plurality of holes 26 during the start-upphase of the fuel reformer 1. If the slits 22 would be radially in linewith the second plurality of holes 28, the main stream of oxidizingagent would stream directly through the second plurality of holes 28,such that the establishment of the desired plug flow is impaired or evenblocked during the start-up phase.

The arrangement of holes is designed in such a way that, in the steadystate of operation of the fuel reformer 1, a homogenous mixture of thehydrocarbon fuel 10 and the oxidizing agent 12 is achieved. The smallersized holes of the first plurality of holes 26 ensure that the stream ofoxidizing agent 12 does not disturb the fuel spray provided by the fuelinjection element 32, and that the main mixing of the hydrocarbon fuel10 and the oxidizing agent 12 takes place with the oxidizing agent 12streaming through the second plurality of holes 28. Thereby, the number,the size and the arrangement of the holes can be optimized for the usedfuel reformer.

A perspective view of a preferred arrangement of the slits and the holesof the first plurality of holes 26 and the second plurality of holes 28is shown in FIG. 3.

FIG. 3 shows a perspective view of a sector having two segments S1 andS2 (indicated by the dotted lines) of the fuel reformer 1, with sidewall 8 a, inner wall 4, outer wall 6 and mixing device 14 (seen from theinside of space 30). As can be seen by the segments S1 and S2 indicatedby the dotted lines in the Figure, the slit 22 and the hole 26 of thefirst plurality of holes are comprised in the first segment S1, whereinthe hole 28 of the second plurality of holes is comprised in the secondsegment S2. It can further be seen in FIG. 2 that the slit 22 isarranged radially off-set to the hole 28 of the second plurality ofholes (meaning that the slit 22 is radially not in line with the hole28), wherein the hole 26 of the first plurality of holes is arranged, inthe same segment S1 as the slit 22 and therefore directly in the radialstream direction of the oxidizing agent 12 streaming through the slit 22into the space 30.

As mentioned above, the size of the holes and slits and theirarrangement is dependent on the overall number of holes. The diameter ofthe first plurality of holes 26 and/or the diameter of the secondplurality of holes 28 are/is defined by the overall number of holes andthe size of the mixing device 14. In the illustrated embodiment, themixing device 14 is divided into segments S1 and S2, the sizes of whichare defined by the overall number of holes, wherein in each segment,preferably alternating, either a hole of the first plurality of holes 26or a hole of the second plurality of holes 28 is arranged. The detailsof these relations are illustrated in FIG. 4.

FIG. 4 shows a schematic front view of a part of the mixing device 14having a radius R, which is mounted at the inner wall 4 of the fuelreformer. Between the inner wall 4 and the outer wall 6, the oxidizingagent passage 18 is defined. In the centre of the mixing device 14, thereceiving opening 40 for the fuel injection element 32 having a radiusRn is arranged. The shape of the receiving opening 40 corresponds to theshape of the fuel injection element 32. Along a first or inner circlewith radius Rd, the first plurality of holes 26 having a diameter d arearranged (in FIG. 4 only a part of said circle with one of these holes26 is shown). Along a second or outer circle with radius RD, the secondplurality of holes 28 having a diameter D are arranged (in FIG. 4 only apart of said circle with one of these holes 28 is shown).

The number of holes ni of the first plurality of holes 26 and the numberof the holes n2 of the second plurality of holes 28 are preferablyequal. The overall number of holes N=n2+ni defines an angle θ by usingthe formula θ=360°/N, wherein θ is defined by the sum of α+β. The anglesα and β are the angles of the segments S1 and S2 each comprising eithera hole 28 of the second plurality of holes or a hole 26 of the firstplurality of holes, respectively, wherein α and β need not to be equal.The angle θ divides the mixing device 14 into equally sized sectors(comprising each a segment of the type S1 and a segment of type S2 asdescribed in FIG. 3), wherein each sector comprises a pair of holes,namely a hole 26 of the first plurality of holes and a hole 28 of thesecond plurality of holes.

According to a preferred embodiment, the size and arrangement of theholes can be defined by the relation that the ratio of the diameter d ofone of the first plurality of holes 26 to the radius Rd of the firstcircle is proportional to tan(β/2) and/or the ratio of the diameter D ofone of the second plurality of holes 28 to the radius RD of the secondcircle is proportional to tan(α/2).

It should be noted that the radius RD of the second circle is alsolimited by the overall size of the mixing device 14 given by the radiusR of the mixing device. The radius RD can be defined by e.g. calculatingthe apothem of a triangle given by the radius R of the mixing device 14and the angle α. The radius RD is then proportional to R according tothe formula RD=R(1−sin(α/2))tan(α/2).

In the embodiment of the invention shown in FIGS. 2-4 the slits 22 arearranged at the segments comprising the holes 26 of the first circle,and their length is proportional to 2ττRβ/360°.

It should be noted in this context that not only two rows or circles ofholes can be arranged at the mixing device 14, but it is also possibleto arrange three or more such rows of holes at the mixing device 14.Also the concentric arrangement is only a preferred arrangement. It isalso possible to arrange the holes homogenously distributed over themixing device 14 (similar to a sieve) or along a planar spiral startingnear the hole 40 for the fuel injection element 32 and continuouslyexpanding on the mixing device 14 outwardly towards the inner wall 4 ofthe housing 2.

The shown embodiment is optimized for reducing recirculation of themixed fluids 10 and 12 due to a repulsion of the mixture from thecatalyst 24, since the illustrated design of the mixing device 14establishes a plug flow in the reaction chamber 16 of the fuel reformer1, which reduces recirculation. For a further improvement of the mixturethe holes of the second plurality of holes 28 can further compriseshrouds (not shown) which provide in the reaction chamber 16 downstreamof the mixing device 14 a swirling motion of the oxidizing agent 12streaming through the holes 28. This swirling motion increases thehomogeneity of the mixture of the hydrocarbon fuel spray 10 and theoxidizing agent 12.

Another advantage of the inventive mixing device 14 is that theoxidizing agent 12 streaming through the holes 26 of the inner circle ishot. Therefore, the heat of the oxidizing agent 12 can also be used toheat the hydrocarbon fuel 10. Thereby, also condensation of the mixedfluids 10, 12 can be reduced.

Even if the mixing device is described in the context of the mixing ofhydrocarbon fuel and an oxidizing agent, the mixing device can also beused for the mixing of other fluids. Another possible application is forexample given as mixing device for the (separate) mixture of air andsteam for providing the air/steam mixture which then in form of amixture is introduced as oxidizing agent into the fuel reformer.

REFERENCE LIST

-   1 fuel reformer-   2 housing-   4 inner wall-   6 outer wall-   8 a, b sides faces-   10 hydrocarbon fuel-   12 oxidizing agent-   14 mixing device-   16 reaction chamber-   18 space=oxidizing agent passage-   20 oxidizing agent supply port-   22 oxidizing agent inlet-   24 catalyst-   26 first plurality of holes-   28 second plurality of holes-   30 space for the oxidizing agent-   32 hydrocarbon fuel inlet-   34 hydrogen rich gas outlet-   36 hydrogen rich gas-   38 preheating means-   40 receiving opening

The invention claimed is:
 1. A planar mixing device for a fuel reformer,the mixing device being arranged for mixing a hydrocarbon fuel and anoxidizing agent, the mixing device comprising a wall comprising anelliptical outer edge and at least a first plurality of holes for theoxidizing agent, the first plurality of holes being arranged along anelliptical first row, and a second plurality of holes for the oxidizingagent, the second plurality of holes being arranged along an ellipticalsecond row, wherein at least one of the first and second rows comprisesholes that are elliptical, wherein diameters of the first plurality ofholes are smaller than diameters of the second plurality of holes,wherein the first and second rows are substantially arranged onconcentric lines, wherein shapes of the concentric lines resemble ashape of the outer edge of the mixing device, and wherein acircumference of the first row is smaller than a circumference of thesecond row.
 2. The planar mixing device according to claim 1, wherein atleast one of the at least two fluids is introduced to the mixing devicein its liquid phase.
 3. The planar mixing device according to claim 1,wherein the first and second rows are formed as concentric circles and aradius of the first circle is smaller than a radius of the secondcircle.
 4. The planar mixing device according to claim 1, wherein thenumber of the first plurality of holes is the same as the lumber of thesecond plurality of holes.
 5. The planar mixing device according toclaim 1, wherein the diameters of the first plurality of holes aresubstantially equal and/or the diameters of the second plurality ofholes are substantially equal.
 6. The planar mixing device according toclaim 1, wherein the diameter of the first plurality of holes and/or thediameter of the second plurality of holes is determined by the overallnumber of holes and the size of the mixing device.
 7. The planar mixingdevice according to claim 1, wherein at least one of a ratio of adiameter of one of the first plurality of holes to a radius of the firstcircle is substantially proportional to tan (β/2) and a ratio of adiameter of one of the second plurality of holes to a radius of thesecond circle is substantially proportional to tan (α/2), wherein theangle β defines a first type of segment (S1) which comprises a hole ofthe first plurality of holes and wherein the angle α defines a secondtype of segment (S2) which comprises a hole of the second plurality ofholes, and wherein n1·α+n2·β is substantially equal to 360°, with n1being a number of holes of the first plurality of holes and n2 being anumber of holes of the second plurality of holes.
 8. The planar mixingdevice according to claim 1, wherein the mixing device is divided intoequal segments, which size is determined by the overall number of holes,wherein in each segment either a hole of the first plurality of holes ora hole of the second plurality of holes is arranged.
 9. The planarmixing device according to claim 1, further comprising a receivingopening which is adapted to accommodate an injection element for one ofthe fluids, particularly a fuel injection element of a fuel reformer,wherein the receiving opening has the same shape as the injectionelement.
 10. The planar mixing device according to claim 1, wherein ateast one hole of at least one of the first plurality of holes and thesecond plurality of holes is equipped with a shroud.
 11. The planarmixing device according to claim 1, further comprising a receivingopening which is adapted to accommodate an injection element forhydrocarbon fuel.
 12. The planar mixing device according to claim 11,wherein the e receiving opening is centrally located in the planarmixing device.
 13. The planar mixing device according to claim 11,wherein the injection element is s fuel injection element for liquidhydrocarbon fuel.
 14. A planar mixing device for a fuel reformer, themixing device being arranged for mixing a hydrocarbon fuel and anoxidizing agent, the mixing device comprising a wall comprising anelliptical outer edge and at least a first plurality of holes for theoxidizing agent, the first plurality of holes being arranged along anelliptical first row, and a second plurality of holes for the oxidizingagent, the second plurality of holes being arranged along an ellipticalsecond row, wherein diameters of the first plurality of holes aresmaller than diameters of the second plurality of holes, wherein thefirst and second rows are substantially arranged on concentric lines,wherein shapes of the concentric lines resemble a shape of the outeredge of the mixing device, wherein a circumference of the first row issmaller than a circumference of the second row, and wherein the mixingdevice is divided into equal segments wherein, in each segment, either ahole of the first plurality of holes or a hole of the second pluralityof holes is arranged, a size of each segment depending upon a number ofholes in the segment.